Bump `dorny/test-reporter` action to 1.6.0

The only reason I'm bothering to do this is that I mistakenly clicked on
one of the execution logs of the "Annotate CI with test results"
workflow and noticed a bunch of deprecation warnings:

	https://github.com/ppy/osu/actions/runs/3610166871

Judging from the release notes [[1]] and diffstat [[2]] of version 1.6.0
of the action affected, a bump should silence these.

[1]: https://github.com/dorny/test-reporter/blob/main/CHANGELOG.md
[2]: https://github.com/dorny/test-reporter/compare/v1.4.2...v1.6.0

.github/workflows/report-nunit.yml

@@ -28,7 +28,7 @@ jobs:
     timeout-minutes: 5
     steps:
       - name: Annotate CI run with test results
-        uses: dorny/test-reporter@v1.4.2
+        uses: dorny/test-reporter@v1.6.0
         with:
           artifact: osu-test-results-${{matrix.os.prettyname}}-${{matrix.threadingMode}}
           name: Test Results (${{matrix.os.prettyname}}, ${{matrix.threadingMode}})